1. Field of the Invention
The invention generally relates to methods and apparatus used in Fiber Distributed Data Interface (FDDI) timed token ring networks to accommodate restricted dialog between preselected stations in the ring. More particularly, the invention relates to methods and apparatus which allow both restricted and nonrestricted token operations to be performed utilizing a hardware protocol implemented by the media access control means associated with each station, coupled with a method of frame linking during restricted operations, wherein said hardware protocol and method of frame linking can be realized without the need for real time software intervention.
2. Description of the Related Art
FDDI token ring networks are well known to those skilled in local area network technology. FDDI is a result of American National Standards Committee X3T9 and grew from the need for high speed interconnection among main frames, minicomputers and associated peripherals. It supports a variety of front-end, back-end and backbone networks configured in a variety of topologies and provides for secure 100 and 200 megabit per second transmission across long distance links (e.g., 100 km), with excellent immunity to the effects of electrical radiation and common mode voltages.
In order to appreciate the context in which the restricted dialog accommodated by the invention takes place, a brief description of the structure of an FDDI token ring network will first be set forth.
At least part of the rationale behind organizing FDDI as a ring is based on the nature of optical communication. Bus and passive star topologies would require the optical transmission to be detected at several sources simultaneously. Although fiber-optic taps are currently becoming available, the optical attenuation caused by such a device would severely restrict the number of nodes on the network.
Fiber-optic communication is still best suited for point-to-point transmission. Two types of Local Area Network (LAN) topologies can be realized with point-to-point links: the active hub star and the ring. Active stars introduce a single failure point that can disable the entire LAN. Single-ring networks also are prone to failures at any node. FDDI alleviates this problem with the dual-ring approach.
An FDDI ring typically comprises a variety of station types. Class A stations connect to both the primary and secondary rings of the network and are often referred to as "dual attachment stations". Data flows in opposite directions on the two rings. The Class A station can act as a wiring concentrator, serving to interconnect several single-attachment or Class B stations to the ring. Wiring concentrators give the network administrator a single maintenance point for a large number of stations. Class B Attachments trade lower implementation costs and ease in servicing against the fault tolerance afforded in a Class A station.
The FDDI defined in X3T9 relates to the lower layers of the Open Systems Interconnection/International Organization for Standardization (OSI/ISO) model as follows:
The lowest layer of the OSI model, the Physical Layer, is described in two documents. The first, the FDDI Physical Medium Dependent (PMD) document, details optical specifications for FDDI. PMD defines the wavelength for optical transmission, the fiberoptic connector employed, and the function of the optical receiver. PMD also details an optional optical by-pass switch that can be incorporated within a station.
The second document describes the FDDI Physical Sublayer (PHY) which is the upper sublayer within the OSI Physical Layer. PHY defines the 4B/5B group-encoding scheme used to represent data and control symbols on the network. PHY also describes the method for retiming transmission within the mode.
The Data Link Layer in the OSI model is often subdivided into two sublayers: Link Layer Control (LLC) and Media Access Control (MAC). FDDI defines the lowest of these sublayers, MAC.
The FDDI MAC protocol distinguishes two classes of service for data transmission; synchronous and asynchronous. Synchronous class guarantees transmission on a per token basis while asynchronous transmission is provided as bandwidth is available on the network. Furthermore, the FDDI MAC protocol provides a restricted token operation mode that allows a limited number of stations to utilize all the bandwidth not reserved for synchronous transmission. These few stations use all this remaining bandwidth to the exclusion of all other stations on the network. Control of the restricted dialog is not defined in the FDDI standards.
Accordingly, it would be desirable if methods and apparatus were available to control restricted dialog on an FDDI token ring network.
Software is one vehicle by which methods could be developed to control parameters that conceptually facilitate restricted token operations. Practically, however, software control of parameters which facilitate the implementation of restricted token operations would be difficult since the software would have to interrupt on token arrivals to manipulate the parameters.
As a result, it would be desirable if a hardware protocol could be developed that would provide reliable control over parameters which could be specifically defined to facilitate restricted token operation. Furthermore, if such a protocol could be coupled with a method for linking frames in buffer memory during restricted operation, restricted dialog could be realized without real time software intervention.
Additionally, it would be desirable if the system designed to facilitate restricted token operation prevented cases where a restricted token is not properly reconverted or an unrestricted token is converted to a restricted type through noise on the ring. The system should at the same time prevent recovery of the ring to its normal unrestricted state prior to completing restricted dialog.